In many applications, it can be crucial for a device to know when zero crossings of an analog signal occur, so that the device can synchronize on the zero crossings. A variety of technical applications and devices can benefit from a determination of zero crossings. Such applications and devices can include phase control, soft switching, power monitoring equipment, timers used in the foregoing, and the like.
A zero crossing is a point at which a signal waveform produced from a digital sampling of the analog signal, which usually appears as a sine wave, changes from having a polarity which is positive to negative or vice versa (more generally crosses an average or DC value). In many cases, however, the analog signal may be contaminated with noise, which can render a determination of the zero crossing difficult.
The noise may be synchronous with the waveform of the signal, such as noise caused by some form of phase control. In other situations, the noise may be asynchronous with the sine wave, such as occurs with noise conducted or radiated from another system.
The noise introduced into the signal can cause the signal to experience a zero crossing, i.e., the noise can cause the signal to cross from being positive to negative or vice versa. Accordingly, a noisy signal can experience multiple erroneous zero crossings.
Conventional filtering techniques, e.g., utilizing a filter, can be used to remove noise from the signal. The changes in amplitude introduced by the noise can be mitigated by such filtering. The filtering, however, typically introduces a phase shift into the signal. Therefore, the time associated with the zero crossing can be obscured by the phase shift.
Sometimes the analog signal is fairly predictable in frequency, as can be experienced in some systems, e.g., using 50-cycle or 60-cycle power. On the other hand, in certain situations, such as with portable power generators, the analog signal can experience frequent changes in frequency. This can introduce additional problems because a phase shift of the filter may vary when the frequency varies.
Some conventional techniques can provide noise rejection via a tracking filter. This, however, can require a high sampling frequency, and in any event, noise introduced into the signal can create multiple zero crossing that obscure the true zero crossing.